Make `future::JoinHandle::abort` actually cancel task

The implementation of `abort` in #87 as detaching the task does not seem right, because detached tasks may still continue to run. Consider the following test, which should not fail but currently does.
```rust
#[test]
fn join_handle_abort_bug() {
check_dfs(
|| {
let (sender, receiver) = futures::channel::oneshot::channel();
let t = future::spawn({
async move {
receiver.await.unwrap();
panic!("should not get here");
}
});
t.abort();
sender.send(()).unwrap();
shuttle::thread::yield_now();
},
None,
);
}
```

(The `yield_now` is needed because otherwise the main task would immediately finish, in which case also the execution finishes because there are no attached tasks left.)

We need a way to actually cancel a task, which drops its continuation and returns a `JoinError` indicating cancellation.

Start at future::JoinHandle::abort and the future::spawn path, then run the supplied join_handle_abort_bug test under check_dfs. Done means abort drops the task continuation so sending on the oneshot cannot reach the panic, and the canceled join returns a JoinError.

Written by the indexing model from the issue text.